Calibration loop. A device used to calibrate induction logging tools. A test loop is an artificial ground loop which consists of a continuous loop (i.e., ring) of electrically conductive material containing a series connected precision resistor. The purpose of the loop is to provide a precise, calibrated, repeatable signal in the induction receiver coils when it is placed over the measure point of the activated tool in a zero signal medium (i.e., air). See induction log.